1. Field of the Invention
This invention relates to a white balance adjusting device in color image sensing apparatus.
2. Description of the Related Art
The conventional white balance adjusting device for color image sensing apparatus is of the automatic follow-up type using an external sensor in colorimetry.
Its circuitry is shown in the block diagram of FIG. 1, comprising a colorimetric sensor 1 receptive of light from a given light source for photo-electrically converting its red, green and blue components, a logarithmic compression circuit 2 for logarithmically compressing the measured values of red, green and blue components by the sensor 1, a differential circuit 3 for subtracting the outputs of the logarithmic compression circuit 2, an image sensing system 5 receptive of light from an object to be photographed for producing R, G and B outputs, amplifiers 6 and 7 for the R and B signals respectively, and a signal processing circuit 8 for producing a certain video signal. For a case where little light is available on the colorimetric sensor 1, to control the R and B signal amplifiers 6 and 7 without using the output of the sensor 1, a constant voltage output circuit 12 is also included along with a switch 13 for changing over between the outputs of the colorimetric sensor 1 and the constant voltage output circuit 12.
In operating the device of FIG. 1, the colorimetric sensor 1 produces output signals S.sub.R, S.sub.G and S.sub.B proportional to the values of the red, green and blue components of the light of the light source. Receiving these signals, the logarithmic compression circuit 2 then produces the outputs proportional to log S.sub.R, log S.sub.G and log S.sub.B. In the differential circuit 3, log S.sub.G is subtracted from log S.sub.R, and log S.sub.B is subtracted from log S.sub.G, to form control signals in the form of voltages for the R and B signal amplifiers 6 and 7 respectively. Then, while the red (R) and green (G) signals from the image sensing system 5 are amplified by the amplifiers 6 and 7, the white balance is adjusted in accordance with the above-identified control voltages.
With such control voltages alone, however, in a situation where the ambient light is so low in illumination to appreciably increase the rating of error in the outputs of the colorimetric sensor 1 and the logarithmic compression circuit 2, an incorrect white balance adjustment results. So, in the conventional device such as that shown in FIG. 1, when the illumination of the ambient light as measured in terms of the log S.sub.G fell below a predetermined reference level, the switch 13 was operated to replace the control voltages derived in the colorimetric sensor 1, logarithmic compression circuit 2 and differential circuit 3 by the constant voltage from the constant voltage output circuit 12. In this case, determination of a value of the constant voltage is made on assumption that the low illumination is from a light source of low color temperature (equivalent to that of candle or tungsten-filament lamp).
The prior art thus minimizes the risk of excessive deviation of the white balance when the ambient illumination is dim.
With such a conventional white balance adjusting device, however, because the fall of the S.sub.G of the sensor 1 as the indicator for the illumination of the ambient light below the certain level makes the control voltage invariantly the constant one, for all ambient lighting situations of lower green component than that level, which may be encountered, the information of the colorimetric sensor 1 has no effect at all. Therefore, if the color temperature of a given ambient light differed markedly from that assumed in setting the constant voltage, a serious problem arose in that the white balance deviated very largely from the ideal one.